1. Field of the Invention
This invention relates to the fabrication of integrated circuit structures, and in particular, to the fabrication of metal conducting lines on the surface of such structures using lift-off wafer processing techniques.
2. Description of the Prior Art
In fabricating integrated circuits and other semiconductor devices, various surface regions of the semiconductor device must be interconnected. The most common method for providing such interconnections comprises forming a layer of an electrically conductive material on the surface of the device, applying a protective mask over those portions of the electrically conductive layer which correspond to the desired pattern of interconnections, and removing those portions of the electrically conductive layer which are not protected by the mask, typically by chemical etching.
Although workable, such conventional processes suffer from a number of disadvantages, particularly when interconnecting exceptionally small regions on the semiconductor device. For example, to ensure complete removal of the unmasked portions of the conductive layer, the pattern must always be at least slightly "over-etched," leading to diminished line widths for a particular line spacing. With very small regions, there is little or no tolerance for such diminished line widths. Although this problem can be lessened by using plasma or reactive ion etching, such processes in turn create undesirable chemical contamination on the surface of the semiconductor device.
As an alternative to such conventional etching techniques, lift-off techniques for forming the desired interconnection pattern on the surface of the semiconductor device have been developed. In general, such lift-off techniques deposit a masking layer directly on the surface of the semiconductor device, exposing only those portions of the surface where it is desired to form the interconnections. By then depositing the conductive material (usually referred to as the metallization layer) on top of the mask, the excess conductive material is blocked from the surface of the semiconductor device by the mask and may be physically lifted by removal of the masking layer. Such lift-off techniques offer superior line width control in the micron and submicron realm because the dimensions of the interconnection pattern are determined by the openings in the mask rather than by the amount of metal removed during an etching process. Moreover, damage to the semiconductor device from radiation is minimized because the device itself is not subjected to plasma or reactive ion etching. A survey of various lift-off processes is presented in "Lift-off Techniques For Fine Line Metal Patterning," by J. Frary and P. Seese, Semiconductor International, Sept. 1981, pp. 72-89.
Such lift-off techniques themselves, however, suffer from certain drawbacks. The most serious drawback is that the metallization layer must be deposited at relatively low temperatures because of poor heat resistance of the materials most commonly used as the lift-off layer. This degrades the electrical characteristics of the devices and lowers the yield of the metalization patterns. To overcome this problem, the use of a polyimide film as the lift-off layer has been suggested. Homma, et al., "Polyimide Lift-off Technology for High Density LSI Metallization," IEE Transactions on Electron Devices, Vol. Ed. 28, No. 5, May, 1981, pp. 552-556.
While the use of a polyimide mask overcomes the limitation on high temperature metallization, it presents certain disadvantages of its own. Removal of the polyimide layer usually requires electrolytic etching since the high temperature metallization renders the polyimide insoluble. Even with such etching, it often difficult to completely remove the polyimide from the surface of the semiconductor device at the end of the fabrication process. Incomplete removal of the polyimide generally results in lower yields by creating short circuits or obscuring portions of the surface of the semiconductor device from subsequent processing operations. It would therefore be desirable to provide a masking material suitable for high temperature metallization which remains soluble in common organic solvents to facilitate complete removal of the material from the surface of the device.